Project Summary: The luminal diameter of muscular arteries and arterioles of the microcirculation is the principal site of control of vascular resistance. These small blood vessels are significant regulators of blood pressure and local blood flow distribution, and impaired control of their luminal diameter is a major contributor to cardiovascular-related diseases. Vascular smooth muscle cell membrane potential depolarization is central to the actions of vasoconstrictors, including intravascular pressure. Despite considerable investigation, the mechanisms underlying membrane potential depolarization remain obscure. In this proposal, we will examine the novel concept that activation of the melastatin transient receptor potential (TRP) channel TRPM4 in arterial myocytes integrates signals activated by vasoconstrictor stimuli to cause membrane potential depolarization. Evidence is provided that activation of TRPM4 channels causes membrane depolarization, Ca2+ influx via voltage-dependent Ca2+ channels, and vasoconstriction. Thus, TRPM4 appears to be an important mediator of smooth muscle cell depolarization. As such, an increase in TRPM4 activity or channel number could contribute to vascular pathologies such as hypertension, which are characterized by smooth muscle membrane potential depolarization and vasoconstriction. TRPM4 therefore presents an attractive target for drug-discovery efforts, but currently, little is known about how activity of the channel is controlled under physiological conditions. Phospholipase C (PLC), protein kinase C (PKC), and intracellular Ca2+ dynamics contribute to the control of arterial tone in vivo, and are thought to be important regulators of TRPM4. This proposal will address the central hypothesis that these factors elicit vasoconstriction, in part, by modulating the activity of TRPM4. Experiments are proposed that will use patch-clamp electrophysiology and live-cell confocal imaging experiments to elucidate how TRPM4 activity is regulated by PLC and PKC activity in vascular smooth muscle cells (Specific Aim 1), and by dynamic Ca2+ events involving IP3-mediated Ca2+ release from intracellular stores (Specific Aim 2). Specific Aim 3 will extend these studies to the level of the intact vasculature and investigate the role of PLC, PKC, and Ca2+-dependent regulation of TRPM4 in pressure and agonist-induced vasoconstriction. These experiments will utilize intact cerebral arteries for intracellular microelectrode recordings of smooth muscle membrane potential confocal Ca2+ imaging, and simultaneous contractile and Ca2+ imaging studies. Additional experiments will employ siRNA technology to suppress TRPM4 expression in intact blood vessels. The outcome of these experiments will significantly enhance our understanding of the role of TRPM4 in blood pressure and blood flow regulation, findings that will contribute to the development of novel therapies for the treatment of cardiovascular-related diseases.